Salmonella typhimurium is an invasive pathogen that causes diseases ranging from mild gastroenteritis to enteric fever. During the infection process, S. typhimurium induces a number of virulence genes required to circumvent host defenses and/or acquire nutrients in the host. We have used the IVET (i&barbelow;n v&barbelow;ivo e&barbelow;xpression t&barbelow;echnology ) system to select for S. typhimurium genes that are induced after invasion of a murine cultured cell line. We have characterized a putative metal ion transporter in Salmonella Pathogenicity Island 1, encoded by sitABCD. The sitABCD operon is induced under iron deficient conditions in vitro and is repressed by Fur. It is also repressed under manganese sufficient conditions in vitro by a manganese dependent regulator, MntR. A proton dependent manganese transporter, mntH is also under the control of these regulators. In addition, a RpoS mediated growth phase dependent regulation of the sit operon was observed. The sit locus is induced in the animal after invasion of the intestinal epithelium. We show that a sit null mutant is significantly attenuated in BALB/c and C3H/HeN mice, suggesting that SitABCD plays an important role in iron and/or manganese acquisition in the animal. Furthermore, a virulence phenotype for the manganese transporter mntH is observed in the animal only in the absence of sit suggesting that these loci possibly transport the same metal(s). Preliminary evidence from monitoring the activities of manganese dependent enzymes in the bacterial cell suggests that Sit is a second manganese transporter of S. typhimurium . Our work on the Sit transporter has yielded interesting insights into the critical role of iron and manganese acquisition in Salmonella pathogenesis.